1. Field of the Invention
The present invention relates to a dielectric resonator having a dielectric core and a cavity, a filter and a duplexer each using the dielectric resonator, and a communication device comprising these devices.
2. Description of the Related Art
A dielectric resonator comprising a dielectric core disposed in a cavity is conventionally used as a small resonator using relatively high electric power in a microwave band.
For example, a dielectric resonator using a TM mode comprises a dielectric core which is made of dielectric ceramic and which is disposed in a ceramic or metal cavity having an electrode film provided on its surface.
FIGS. 10, 11A and 11B show an example of the construction of a conventional dielectric resonator. FIG. 10 is an exploded perspective view, and FIGS. 11A and 11B are a top view and a sectional view, respectively. In this example, a dielectric core 3 having electrodes formed on each of two opposing side faces is inserted into a metal cavity body 1, and both side faces are soldered to the inner surface of the cavity body 1 with solder 6. Also, a cavity cover 2 is provided on an opening of the cavity body 1.
In this structure in which both side faces of the columnar dielectric core are joined to the inner wall of the cavity, when the dielectric core and the cavity have a large difference between coefficients of linear expansion, the joints between the dielectric core and the cavity deteriorate due to heat cycle fatigue, causing a problem in which sufficient reliability cannot easily be obtained.
Therefore, another structure is used in which a dielectric core and a cavity are integrally molded. In this structure, the dielectric core and the cavity are made of the same ceramic material, and thus the problem of heat cycle fatigue does not occur.
However, in the structure in which the dielectric core and the cavity are integrally molded, most of the cavity, which need not have dielectricity, is formed by using a dielectric ceramic material, and thus the material cost is increased. Furthermore, the mold used becomes complicated and increases the production cost.
The applicant filed Japanese Application No. 11-283037 relating to a resonator device comprising a conductive bar and a dielectric core, both of which are provided in a cavity, so as to use both a resonance mode and a coaxial (semi-coaxial) resonance mode due to the dielectric core. However, the cavity made of a general metal material such as aluminum and the dielectric core have a large difference between the coefficients of linear expansion, and thus sufficient reliability cannot be achieved for the joints between the dielectric core and the inner wall of the cavity. If a metal material having substantially the same coefficient of linear expansion as the dielectric ceramic material of the dielectric core is used for forming the cavity, the above problem can be resolved, but there is the problem of increasing the material cost of the cavity and the production cost required for processing the cavity.